Chapter 5 Brief history of climate: causes and mechanisms Climate system dynamics and modelling Hugues Goosse
Outline Investigation of the role of the external forcing and of the internal dynamics. Analysis of key periods to illustrate dominant processes. Chapter 5 Page 2
Forced and internal variability Forced variability: driven by changes in external forcing Internal variability: caused by interactions between various elements of the system Chapter 5 Page 3
Forced and internal variability Forced variability: possible to find the ultimate cause of the observed changes Internal variability: only the chain of events can be identified, the proximate cause . Chapter 5 Page 4
Forced and internal variability The climate system is sensitive to small perturbations. Chapter 5 Page 5
Forced and internal variability The climate system is sensitive to small perturbations. Consequences: 1. The skill of weather forecasts is limited in time. 2. Two simulations include different realisations of internal variability. 3. An agreement between simulations and observations on the timing of unforced events is not expected on the long term. Chapter 5 Page 6
Forced and internal variability The magnitude of internal variability is a strong function of the spatial and temporal scale investigated. Median of the standard deviation of the annual mean surface air temperature from control simulations performed in the framework of CMIP5. Figure from E. Hawkins updated from Hawkins and Sutton (2012). Chapter 5 Page 7
Timescales of climate variations The timescale of climate variations is set up by both the forcing and internal dynamics. Schematic representation of the dominant timescales of selected external forcing and processes related to internal dynamics which affect climate. Chapter 5 Page 8
El Niño-Southern Oscillation In normal conditions, the thermocline is much deeper in the West Pacific than in the East Pacific. Figure from Christensen et al. (2013) In El Niño conditions, the intensity of the upwelling is reduced in the East Pacific and the SST warms in the East Pacific. Chapter 5 Page 9
El Niño-Southern Oscillation The Walker circulation is associated with a positive feedback, called the Bjerknes feedback. Chapter 5 Page 10
El Niño-Southern Oscillation The atmospheric circulation and sea surface temperature exhibit irregular oscillations: El Niño Southern Oscillation (ENSO). Time series of the temperature in the eastern . equatorial Pacific (averaged over the area 5 ° N-5 ° S- 170 ° W-120 ° W, the so-called Niño3.4 index) and the SOI index (normalized difference between SLP in Tahiti and Darwin). Source: http://www.cpc.ncep.noaa.go v/data/indices/ Chapter 5 Page 11
El Niño-Southern Oscillation ENSO is also associated with nearly global scale perturbations . . Correlation between the sea surface temperature in the eastern tropical Pacific (Niño3.4 index) and sea-level pressure in January. Chapter 5 Page 12
The North Atlantic Oscillation The mid-latitude westerlies in the North Atlantic present irregular changes in their intensity and in the location of their maximum. Correlation between the winter NAO index and the winter SLP (average over December, January, February). Chapter 5 Page 13
The North Atlantic Oscillation The NAO is associated with changes in many atmospheric and oceanic variables. Correlation NAO index-winter temperatures Correlation (top) and regression in ° C (bottom) between the winter NAO index and the winter surface air temperature (average over December, Regression NAO index-winter temperatures January, February). Chapter 5 Page 14
The Atlantic multidecadal oscillation and the Pacific decadal oscillation The sea surface temperature is characterized by pronounced decadal and multidecadal variations. Regression between PDO and AMO indices with annual sea surface temperature. Figure from Hartmann et al. (2014). Chapter 5 Page 15
Reconstructing past climates Past climate variations can be reconstructed using the signal recorded in natural archives by various sensors. Schematic illustration of the forward and inverse approaches. Chapter 5 Page 16
Reconstructing past climates Dating methods Annual layer counting. 5 cm-long section from the lake sediment of Cape Bounty, East Lake, Nunavut, Canada. Picture from François Lapointe. Chapter 5 Page 17
Reconstructing past climates Dating methods Radiometric dating: based on the decay of radioactive isotopes. decay constant The decay follows a standard law: of the radioactive N e R t N isotope 0 concentration initial of radioisotopes concentration at time t at time t =0 Chapter 5 Page 18
Reconstructions based on isotopes Oxygen isotopes The abundance of isotopes is measured using the delta value. 18 16 / O O sample 18 O 1 .1000 18 16 O / O standard 18 O is the ratio of 18 O and 16 O isotopes in the sample, compared to a standard. Chapter 5 Page 19
Reconstructions based on isotopes Oxygen isotopes Isotopic fractionation takes place during evaporation and condensation Chapter 5 Page 20
Reconstructions based on isotopes Carbon isotopes During photosynthesis, 12 C is taken preferentially to 13 C because it is lighter. 13 12 C / C sample 13 C 1 .1000 13 12 / C C standard Organic matter has a low (negative) 13 C. Chapter 5 Page 21
The Climate since the Earth’s formation The uncertainties on Earth’ climate are larger as we go back in time. A simplified geological time scale.
Precambrian climate 4 billion years ago, the solar irradiance was about 25-30% lower than at present but the Earth was not totally ice covered : the “faint young Sun paradox”. Main hypothesis: a much stronger greenhouse effect caused by a much higher CO 2 (250 times the present-day value?) and CH 4 concentration. Chapter 5 Page 23
Precambrian climate Atmospheric composition has been modified with time. The photosynthesis induced a large increase in the atmospheric oxygen concentration 2.2. to 2.4 billion years ago. This caused a glaciation ? Chapter 5 Page 24
Precambrian climate Large glaciations took place around 550 to 750 million years ago. Formation of a Snowball Earth around 635 million years ago? If this is really occurred, why does not Earth not stay permanently in this state ? Chapter 5 Page 25
Phanerozoic climate The carbon cycle and climate appear strongly linked on timescales of millions of years. Changes in atmospheric CO 2 concentration can be represented by: CO 2 Volc t ( ) Weath t ( ) Org t ( ) t Silicate Outgassing of Long-term weathering and CO 2 due to burial of calcium metamorphism organic matter carbonate and volcanic sedimentation eruptions in the ocean Chapter 5 Page 26
Phanerozoic climate The models based on this balance are able to reproduce the long term changes in the carbon cycle. Large influence of climate sensitivity Comparison of the CO 2 concentration calculated by GEOCARBSULF model for varying climate sensitivities (noted D T(2x) on the figure) to an independent CO 2 record based on different proxies . Figure from Royer et al. (2007). Chapter 5 Page 27
Cenozoic climate The temperature over the last 65 million years has gradually decreased . This is associated with a cooling that is often referred to as a transition from a greenhouse climate to an icehouse. The global climate over the past 65 million years based on deep-sea oxygen-isotope measurements. Figure from Zachos et al. (2008). Chapter 5 Page 28
Cenozoic climate During the Paleocene Eocene Thermal Maximum (PETM, 55 million years ago) global temperature increased by more than 5°C in about 10 000 years. Carbonate carbon isotope and oxygen isotope ratio in two cores in the South Atlantic. The time on the x axis starts at the onset of the PETM about 55 million years ago. Figure from McInerney and Wing (2011). Chapter 5 Page 29
Cenozoic climate 50 million years ago, the location of the continents was quite close to that of the present-day one but changes in boundary conditions still had an influence on climate. Land configuration about 60 million years ago. Map from Ron Blakey Chapter 5 Page 30
Cenozoic climate Large climate fluctuations have occurred over the last 5 million years. Benthic 18O, which measures global ice volume and deep ocean temperature. Data from Lisiecki and Raymo (2005). Chapter 5 Page 31
The last million years: glacial interglacial cycles The characteristics of the Earth’s orbit are determined by three astronomical parameters. obliquity ( e obl ), eccentricity ( ecc ) Chapter 5 Page 32
The last million years: glacial interglacial cycles The climatic precession is ecc sin (PERH) = ecc sin ( w ) Chapter 5 Page 33
The last million years: glacial interglacial cycles Because of the climatic precession, the Earth was closest to the sun during the boreal summer 11 ka ago and the closest to the sun during boreal winter now. Chapter 5 Page 34
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